Thompson Belly Reducer

About This Item

Excessive soundboard "bellying," caused by string tension, can make an acoustic guitar difficult to play. It can even damage the top, bridge plate and bridge. Replacing the original bridge plate to correct the problem is a time–consuming job that can unfortunately reduce the value of a vintage instrument.

Respected luthier and restoration expert TJ Thompson developed a simple tool set, the Thompson Belly Reducer, that can fix soundboard bulging without permanent alterations to the top or bridge plate.

The Thompson Belly Reducer is a set of five precision-machined aluminum tools: convex flatteners for vintage–style belly and rectangular bridges, and concave flatteners for the three most common bridge plate shapes.

Aligning the cauls is now easier—magnets are embedded right into the tools. Magnetic cauls stay in place (hands free!) while installing and tightening the clamps.

They're designed for use with the bridge removed from the instrument, and are effective for bridge plates that have been installed with hide glue, aliphatic (yellow) glue, or PVA (white) glue. Simply match the appropriate flatteners to your guitar, heat them to 150 degrees F, dampen the wood on the bridge area and bridge plate, and clamp the flatteners into place for a day or so.

More clamping solutions from T.J. Thompson

Video

Trade Secrets!

Trade Secrets!

Dan Erlewine's got a Martin with a potbelly: string tension has pulled the top into a hump on this 1928 0-18. To make things tougher, the guitar top's encased in a thick layer of epoxy finish! It's an interesting puzzle...

Customer Reviews

Just OK

I volunteered my ability to help restore instruments that were damaged in the Harvey storm.It was important that I keep the original bridge plate and although pretty high priced,I bought it.The magnets were not strong enough and I ended up having to hold the underside to get it in place.I was restoring a 1945 Martin 00 17 ,same thing Dan does the demo on.He didn't have trouble with his magnets.The belly flattened somewhat but this tool needs to go back to the drawing board.

magnets

I purchased this with the intention of using them when a job came in . Really keen to try the magnets.left me flat , must be a really thick top and plate as they may as well not be there. A local luthier has made an electric heater for his set. put them in place and turn it on

This works great!

The local guitar shop owner (who subs out repair work to me) presented an early Seagull dreadnaught with the bridge hanging by a thread, and a very pronounced belly underneath it. I ordered this product and in 24 hours the belly had not only been flattened, it became slightly concave (I overtightened the clamps.) So I flipped the plates (flat sides towards the bridge), wetted it again, and gave it another 24 hours, which made it flat under the bridge. Next time I won't tighten the clamps so tight on the first pass.

The magnets worked for me, using the small trapezoid underneath, but the other plates are larger, and your top may be thicker, so YMMV. However, a trick to beef up the magnets is to get four N52 neodymium 1/4"x1/4" cylinders, and stack one atop each of the embedded magnets (pictured), which will essentially essentially double the attractive force.

Buy It!

A customer had left a Martin DM hanging in a wood-stove-heated room for a few years. The bridge had lifted off and there was bellying behind the bridge. Normally, I reattach the bridge then toss in a bridge doctor if there's normal bellying. In this case the top was so warped getting a reliable glue joint was doubtful. The same day I started the repair, Stew-Mac sent out an email blast featuring the Thompson Belly Reducer (timing is everything). I had it in my shop two days later and two days after that I had a nice level surface in which to attach the bridge. This one repair alone almost paid for the tool.

- Steve Beckwith

Rescued a classic!

Been meaning to get to this for some time. I second all the positive reviews, and my set was the older one, bought in 2011 when I had one of the infamous "forward x-brace" pre-war D28s Herringbones. I understand that though these sound nice, they were prone to top warping. A guitar which was unplayable when we received it netted the seller $35k. The plates paid for themselves on the first use.

Well-designed, very useful tool

I bought this after a customer brought in a 1959 small body Guild steel string with a mahogany top with a very bad belly bow. I used the reducer three separate times over a week with final excellent results. It more than paid for itself on the first job, so I am happy to have this tool in my chest for future use.As far as the strength of the magnets mentioned in the previous comment I found that they had plenty of power to keep the pieces on the top without a clamp or other support but due to their small size had to be exactly lined -up before they would connect to each other.

A Most Excellent Tool

I've been using these tools for quite some time now on different model guitars with bellying problems and i'm very happy with them. i have the early models without magnets. To position the inside one i insert it cold and mark the desired spot with a permanent marker through two bridgepin holes. I then prepare a wooden post with a string attached to it that will temporarily hold the inside caul once it's heated up. The string hangs outside the sound hole. i then place the guitar face down with a moistened sponge on the bridgeplate for at least an hour, while i heat up the cauls. Once they're hot enough and the bridge plate glue is malleable i insert the inside caul with the wooden post which gives me time and space to place the top caul and insert the clamp. pull the string to remove the tempoarary post before tightening the clamp!

Thompson Belly Reducer

Bit of a pig to use if you have big hands (like me) and can't get both your arm and the first clamp into the soundhole at the same time but if you can get it in place whilst it is still hot and the wood wet, it works a dream! Worth persevering with, it really does work. All else fails, try getting someone to hold the guitar soundhole down to get the hot inside caul in place.

Thompson Belly Reducer

This tool is a great concept; however, using it is a puzzle. The video interview does not explain use and nowhere are there any instruction on how to apply this. It would be great if Dan or Erick would make a brief video on how to properly use this tool.